Image scanner for electron microscopes



Aug. 28, 1962 M. E. HAlNE ETAL IMAGE SCANNER FOR ELECTRON MICROSCOPES Filed Nov. 24, 1958 ll I ivv PETER H A mu w AI f mmm m itinguish. to resort to photographing the image to obtain the full detail.

United States Patent 3,051,860 Patented Aug. 28, 1962 ice ' 3,051,860 'IMAGE SCANNER FOR ELECTRON MICROSCOPES .Michael Edward Haine, Sulhamstead, and Peter Alex Einstein, Tilehurst, Reading, England, assignors to Associated Electrical Industries (Manchester) Limited, a British company Filed Nov. 24, 1958, Ser. No. 776,012

Claims priority, application Great Britain Nov. 27, 1957 2 Claims. (Cl. 313-65) The present invention relates to electron microscopes and in particular to apparatus for intensifying images thereof.

In electron microscopes the ,final image is normally produced by the electrons falling on a fluorescent screen and an image so formed is usually very dim owing to the fact that the high magnification in the microscope results in a low electron current density proportional to the inverse square of the magnification. To enable the operator to see such an image it is necessary for him to adapt his eyes to the dark but even then the detail of the image, which is important to the operator, is very hard to dis- It is therefore necessary in most microscopes However, it is still necessary to examine the image with the naked eye for selecting the areas to be g phot-ographed, focussing the image etc.

The object of the present invention is to provide an energy electrons in accordance with a raster, the arrangement being such that when said first surface is irradiated with an image carrying beam of high intensity electrons, .the conductivity of each portion of said photo-conducting fplate is increased in accordance with the intensity of the "radiation thereon, so that when each successive portion of said second surface is charged by said low energy electron beam a current flows through said plate to said conducting layer, together with means for deriving signals corresponding to the instantaneous value of said current and means for applying said signals to a cathode-ray tube so as to produce a picture corresponding to said electron image. I V V 0 g 1 Preferably, the photo-conducting material is selenium. The seleniumisconveniently supported by a thin insulating plastic membrane on each of the two surfaces of which a thin film of electrically conducting material has been deposited.

The term photoconducting materia as used in this description is taken to mean any material the electrical conductivity of which is increased by irradiation by an electron beam.

In order that the invention may be more readily understood reference will now be made to the acompanying drawing in which:

FIG. 1 is a diagram showing the relative positions of the principal components of the aparatus, and

FIG. 2 is a cross-sectional view on an enlarged scale of the screen of the apparatus.

With reference to FIGS. 1 and 2, the apparatus comprises a composite screen forming an intensifying plate which is constructed as follows. A thin supporting membrane 2, for example, of a plastic material, is stretched tightly over an annular frame 3.

Each surface of the membrane 2 is coated with a very thin film of metal, such as aluminium, having a thickness conducting base electrode.

of the order of 1000 A., which renders each surface conducting a-nd the membrance therefore forms a suitable The metal films carry the reference numerals 4 and 5 in the figure.

On one of said metal films 5 a film of photoconductive material 1 is deposited. This may be amorphous selenium and may be deposited by any of the known techniques of evaporation. On the free surface of the selenium layer 1 a film of material which is anisotropically conducting may be deposited. Such a material may be formed by the deposition of a carbon or metallic smoke, such as gold, and the layer of material will have low conductivity parallel to the surface of the selenium film and a high conductivity in a direction norm-a1 to said surface.

The metal films 4 and 5 and the membrane 2 are together sufficiently thin as to be opaque to visible light but substantially transparent to high energy electrons.

An electron gun -7 emits an electron beam 8 which is accelerated to an energy of a few hundred volts and is 20.

across the film in a raster, such as in a television system,

by coils 9.

The metal film 5 is connected through a resistor 10 to a source of electric potential 11 which is at a few volts positive to the cathode of the gun 7. Metal film 4 is earthed through a high resistance 17, this preventing the upper surface of membrane 2 from acquiring undesirable charging-up potentials resulting from stray high voltage electrons from the primary electron beam.

When the intensifying plate described above is used in combination with an electron microscope, the whole as- .sembly of the intensifying plate and the electron gun is inserted in the envelope of the electron microscopeso that the beam of high energy electrons carrying an image in the electron microscope will impinge directly upon the free surface of the film 4. The envelope is subsequently evacuated and with this arrangement there is no pressure difference across the intensifying plate.

The operation of the apparatus is as follows:

The selenium film 1 is irradiated with the low energy electron scanning beam 8 and since the conducting film 5 is at a potential higher than the cathode of the electron .gun 7 the free surface of the film 1 will be charged up of this film is normally very high this current will be negligible.

The high energy electron image beam 12 from the electron microscope impinges upon the free surface of the film 4 and is transmitted through the membrane 2 and film 5 onto the selenium film 1 without appreciable attenu ation. The electron image beam then penetrates the selenium film and in doing so renders it more conductive. This increase in conductivity in any particular portion of the film will depend upon the intensity of the portion of said electron beam impinging on that portion of the film as Well as the potential difference across the film, so that parts of the selenium film that would appear brightest in a normal image, ie in an image that would be produced if the selenium film were replaced by a fluorescent screen, will have the greatest conductivity.

The charge deposited on each portion of the free surface of the selenium film by the low energy electron scaning beam 8 will leak away between one scan of the charging raster and the successive one by an amount depending on the increased conductivity induced in each portion of the film by the high energy electron image beam 12. Therefore when the scanning beam returns to a particular portion it will deposit thereon a charge sufficient to return the surface potential thereof once more to that of the cathode and, so doing, will induce by capacitive action a similar charge on the conducting film 5. This charge will pass through the load resistor generating a potential pulse which may be led away through a capacitor as a signal to an amplifier and thence to a cathode ray oscillograph scanned in a raster synchronously with the raster of the beam 9. The signal will be applied to the grid of this cathode ray tube and thus a picture may be produced corresponding in intensity with the intensity of the electron microscope image beam but with an overall increase in intensity as required.

It is found beneficial to the operation of the apparatus to include electrostatic guard rings 13 and 14, as shown, on the side of the selenium film which is scanned by the low energy electron scanning beam, these guard rings are situated adjacent to the selenium film but are insulated therefrom. The potentials of these guard rings have values of the order of +3 volts and +50 volts respectively and their purpose is to prevent undesirable effects caused by the leakage of charges from the signal plate 5 onto the front of the selenium film and also caused by stray electrons scattering while the scanning beam is located near the extremity of its traverse.

It has been shown experimentally that the effect of irradiating a film of selenium with a high voltage electron beam, such as is used in the electron microscope, will produce a conductivity whereby between 1000 and 5000 electrons pass across the film for every one high energy electron entering.

Under these circumstances it is possible to calculate that every 100 electrons falling on a small image area corresponding to about microns in diameter will produce sutficient conductivity in the film to reduce the potential across it by about 1 to 10 volts. This is sufficient to give a good signal on the signal plate when this area is scanned by charging beam. It should thus be possible to produce an appreciable contrast for each image point providing it is irradiated by at least 100 electrons between one scan and the next. Since in any case at least 100 electrons are necessary per image point in order to reduce random noise fluctuation, the proposed image tube system should produce a picture as good as is theroetically useful.

It will be appreciated that the photo-conductive film need not necessarily be selenium and other suitable photoconductive material may be used.

The apparatus described provides a method of increasing the intensity of an electron microscope image and thereby rendering the details thereof more easily visible to the operator.

In a modified form of operation of the apparatus of the invention the film 5 is kept at a potential which is a few hundreds volts positive with respect to the cathode of the low energy electron emitting gun 7, and the free surface of the photo-conducting film 1 stabilises at the secondary emission cross-over point, i.e. the potential where, when the electrons hit it, the surface emits exactly one electron for each impinging electron.

The conducting film 4 is described as being connected through a high resistance 17 to earth. As an alternative it could be connected directly to the film 5.

What we claim is:

1. For use in electrical apparatus for intensifying an electron image, an intensifying plate comprising an annular supporting frame, a thin tightly stretched plastic supporting membrane stretched tightly across said supporting frame and having an accurately flat surface, a thin film of electrically conducting material which is transparent to the passage of high energy electrons coated onto said fiat surface, means for connecting said conducting film to a source of electric potential, and, on the free surface of said conducting film, a sensitive layer of a material the electrical conductivity of which is increased by irradiation by an electron beam, the arrangement being such that when the free surface of said supporting membrane is irradiated with an image carrying beam of high energy electrons, the electrical conductivity of each elemental portion of said sensitive layer is varied in accordance with the intensity of the electrons falling thereon.

2. For use in electrical apparatus for intensifying an electron image, an intensifying plate comprising an annular supporting frame, a thin tightly stretched plastic supporting membrane stretched tightly across said supporting membrane and having an accurately flat surface, a thin film of electrically conducting material which is transparent to the passage of high energy electrons coated onto said fiat surface, means for connecting said conducting film to a source of electric potential, on the free surface of said conducting film a sensitive layer of a material the electrical conductivity of which is increased by irradiation by an electron beam, and on the surface of said sensitive layer a coating of a material which is conducting in a direction normal to the surface of said sensitive layer but is insulating in a transverse direction, the arrangement being such that when the free surface of said supporting membrane is irradiated with an image carrying beam of high energy electrons, the electrical con ductivity of each elemental portion of said sensitive layer is varied in accordance with the intensity of the electrons falling thereon.

References Cited in the file of this patent UNITED STATES PATENTS 2,555,091 Lubszynski May 29, 1951 2,654,853 Weimer Oct. 6, 1953 2,804,561 Sheldon Aug. 27, 1957 2,820,921 McGee et al Jan. 21, 1958 2,878,416 Theile Mar. 17, 1959 

